Grenacher's type of larva, where the external yolk-sack is practically absent, appears to me to lend confirmation to this view. If the reader will turn to fig. 113, he will observe a prominence between the mouth and anus, which exactly resembles the ordinary Gasteropod foot. At the sides of this prominence are placed the rudiments of the arms. This prominence is filled with yolk, and represents the rudiment of the external yolk-sack of the typical Cephalopod embryo. The blastopore, owing to the smaller bulk of the food-yolk, reverts more nearly to its normal position on the oral side of this prominence. If the above considerations have the weight which I attribute to them, the unpaired part of the Cephalopod foot has been overlooked in the embryo on account of the enormous dilatation it has undergone from being filled with food-yolk; and also owing to the fact that in the adult the median part of the foot is unrepresented. The arms are clearly, as Huxley states, processes of the margin of the foot. Both Grenacher and Huxley agree in regarding the funnel as representing the coalesced epipodia; but Grenacher points out that the anterior folds which assist in forming the funnel (_vide_ p. 253) represent the great lateral epipodia of the Pteropod foot, and the posterior folds the so-called horse-shoe shaped portion of the Pteropod foot. _Development of Organs._ The epiblast. With reference to the general structure of the epiblast there is nothing very specially deserving of notice. It gives rise to the whole of the general epidermis and to the epithelium of the organs of sense. The most remarkable feature about it is a negative one, viz. that it does not, in all cases at any rate, give rise to the nervous system. The epiblast of the mantle has the special capacity of secreting a shell, and the integument of the foot has also a more or less similar property in that it forms the operculum, and a byssus in some Lamellibranchiata, other parts of the integument form the radula, setæ in Chiton, and other similar structures. Nervous system. The origin of the nervous system in Mollusca is still involved in some obscurity. It is the general opinion amongst the majority of investigators that the nervous ganglia in Gasteropods and Pteropods are formed from detached thickenings of the epiblast. Both Lankester (No. 239) and Fol (No. 249-251) have arrived at this conclusion, and Rabl has shewn by sections that in Planorbis there are two lateral thickenings of the epiblast in the velar area; from which the supra-oesophageal ganglia become subsequently separated off. The observations on the pedal ganglia are less precise: they very probably arise as thickenings of the epiblast of the side of the foot.